This invention relates to a heat resistant resinous adhesive composition. The invention further relates to an adhesive film and a composite film utilizing the adhesive composition.
Heretofore, flexible circuit-printable boards have been produced, for instance, by combining a metal film such as a copper foil and a heat resistant polymer film such as an aromatic polyimide film by an adhesive such as an epoxy resin or a urethane resin. Since these known flexible circuit-printable boards are not sufficiently satisfactory in their heat resistance, they sometimes suffer troubles such as swelling and peeling-off at the layer of an adhesive when they are placed under high temperature conditions, for instance, they are brought into contact with a melted solder. Therefore, a number of studies for improvement of heat resistance of the adhesives have been made. Similar problem occurs in a metal-foil laminated substrate for tape automated bonding (TAB).
For instance, an imide resin-type adhesive such as a precondensate of N,N'-(4,4'-diphenylethane)bismaleimide and diaminophenylmethane has been proposed. This adhesive, however, is not appropriately employed as an adhesive for flexible circuit-printable boards, because a cured product of the adhesive is poor in its mechanical strength.
Japanese Patent Provisional Publications No. 62(1987)-232475 and No. 62(1987)-235382 describe an adhesive film (i.e., a dry film) of a resinous composition comprising an aromatic polyimide prepared from benzophenonetetracarboxylic acid and an aromatic diamine and polybismaleimide. This film is used in such manner that the film is placed between a heat resistant film such as a polyimide film and a copper foil and they are heated under pressure to give a composite sheet. This adhesive film has a high softening point such as a softening point of not lower than 180.degree. C. Therefore, the adhesion of a polyimide film and a copper foil should be done at a high temperature of approx. 260.degree. to 280.degree. C. and at a high pressure of approx. 30 to 60 kg/cm.sup.2. Such conditions are very severe for conventionally employed heat-pressure rolls made of an organic resin. Accordingly, it is extremely difficult to laminate a polyimide film on a copper foil in a continuous procedure.
As coating compositions for coating electronic parts such as circuit-printable boards, a number of resin solutions (i.e., varnishes) comprising an aromatic polyimide and an epoxy resin have been proposed. However, these resin solutions are not practically employable as adhesives for combining a metal film such as a copper foil and a heat resistant film, particularly, an aromatic polyimide film, because these adhesives have drawbacks such that the adhesion procedure or curing procedure should be performed at extremely high temperatures, the aromatic polyimide is hardly miscible with the epoxy resin, the aromatic polyimide is hardly dissolved in a solvent used, and/or a cured adhesive layer is poor in its flexibility.